In recent years, as communications speed up, frequencies used for mobile phones and so on have increased to the 1 GHz, 2 GHz, and 5 GHz bands. Thus, a technique for generating an RF frequency is important. As a solution for attaining the technique, a technique of multiplying a low frequency of the original signal to a desired frequency is available.
Doublers frequently used as integrated circuits include an application of a Gillbert cell double-balanced mixer circuit, which uses a differential amplifier shown in FIG. 3. In this circuit, high frequency differential signals having been phase shifted from each other by 90 degrees are inputted to inputs RFin1 and RFin2, respectively, and a doubled signal is obtained via a resistance load. This operating principle will be briefly discussed below. The inputs having been phase shifted by 90 degrees can be represented as a sin wave and a cos wave. The frequency conversion of a double balanced mixer is expressed by formula 1 below.y=sin f*cos f=½ sin 2f  Formula 1where f represents a frequency of an inputted signal. As a result, a frequency twice as high as a frequency of the original signal is generated from formula 1 as an output.
It is understood that a doubled signal can be obtained thus by the double balanced mixer. When a phase difference between input signals is deviated from 90 degrees, formula 1 is not established and a frequency obtained by adding a frequency of an input to an output, that is, a double frequency is generated and a DC component, which is a frequency difference between inputs, is also generated. This means that the output level of a signal with a double frequency is also changed, resulting in a problem in practical use. In actual circuits, a phase-shift circuit and a correction circuit for obtaining a preferred 90-degree phase shift are used to prevent such a deviation of a phase difference. However, the circuit size is increased by using such a circuit and current consumption also increases. Meanwhile, a simple technique for using a resonance circuit for an output is also proposed.
Referring to FIG. 4, the following will describe a method disclosed in JP2998773B where a transistor circuit and a resonance circuit are combined. In this circuit, a high frequency signal inputted to the base of a transistor Q is basically distorted by the transistor Q which is biased to increase nonlinearity. The high frequency signal is selectively amplified by a double resonance circuit (L, C2) connected to a collector.
However, the circuit of JP2998773B shown in FIG. 4 is a single-phase circuit and thus is not applicable to an integrated circuit using a differential circuit. Moreover, in this circuit, the output of the original signal serving as an input signal is just suppressed by the frequency characteristic of the resonance circuit connected to the collector. Thus, the circuit is not applicable to a system requiring a high suppression level of the original signal. Meanwhile, in the doubler which is an application of the Gillbert cell double-balanced circuit using the differential amplifier of FIG. 3, the component of the inputted original signal is offset and is not outputted in theory. In reality, an ideal multiplication is not performed due to asymmetry caused by variations between transistors. The original signal is outputted but has an extremely small value as compared with the circuit configuration of FIG. 4. However, as described above, inputted signals are phase shifted by 90 degrees and thus the output levels of an outputted DC and a double wave are changed.